With recent developments such as those in the Internet, networks have been spreading to both homes and offices. Such a network is called a LAN (local area network). A wireless LAN that is a wireless version of the LAN has also been spreading. For example, as a high-speed wireless access system using a 2.4 GHz band or a 5 GHz band, spread of a wireless LAN or the like of the IEEE (Institute of Electrical and Electronics Engineers) 802.11g standard and the IEEE 802.11a standard is remarkable. In these systems, a transmission rate (a transmission rate in a physical layer) of a maximum of 54 Mbit/s has been achieved using an orthogonal frequency division multiplexing (OFDM) modulation scheme, which is technology for stabilizing the property in a multi-path fading environment.
However, the transmission rate cited here is a transmission rate on a physical layer. In fact, since the transmission efficiency in a MAC (medium access control) layer is about 50 to 70%, an upper limit of the actual throughput is about 30 Mbps and this property is further deteriorated when communication parties requiring information increase. On the other hand, in the world of a wired LAN, FTTH (fiber to the home) using an optical fiber, including a 100 Base-T interface of the Ethernet (registered trademark), has been spreading to individual homes, and thus provision of a high-speed line of 100 Mbps has been spreading. Even in the world of wireless LANs, a higher transmission rate is required.
Therefore, as a standard for realizing a higher transmission rate, there is IEEE 802.11n standard. In this standard, a transmission rate of a maximum of 600 Mbit/s can be realized by using MIMO (multiple input multiple output) technology as spatial multiplexing transmission technology.
In recent years, in order to achieve a larger capacity, wireless communication using MU (multi user)-MIMO that enables wireless space resources to be effectively used by performing one-to-many communication through spatial multiplexing between a wireless base station and a plurality of wireless terminal stations using the same wireless channel has been studied as a wireless system using MIMO technology (see, for example, Non-Patent Document 1). For example, an MU-MIMO transmission method has been studied in IEEE 802.11ac (see Non-Patent Document 2).
In MU-MIMO, channel information between antennas of a base station and each of the terminals is estimated; the base station acquires the channel information, controls transmission beams using the acquired channel information, and directs the beams toward the terminals in optimal directions, thereby making it possible to simultaneously transmit signals to the terminals using the same frequency channel.
That is, control has been performed to increase directivity to wireless stations that are transmission destinations and to direct nulls to the other specific wireless stations. Since spatial multiplexing is performed, it is also called a space division multiple access (SDMA) control scheme. Since communication can be simultaneously performed in the same frequency and at the same time, it is possible to increase the spectral efficiency.